Just don't know enough about the situation to comment one way or another.
And yes, volunteers do have a choice to respond.
There have been times that I have been in the station, and for whatever reason - fatigue, having to get up early for work or simply having to be somewhere in a short period of time, have not responded to a run, including a structure fire or two. There are simply times that you have very valid reasons for not responding.
Don't know what the situation here was there simply isn't enough information provided.